The present invention relates to a compressed air supply system for a utility or commercial vehicle, with a compressor drivable by a drive via a pneumatically switchable coupling, and with a valve having a magnet coil, for the selective supply of compressed air to a switch input of the coupling.
Compressed air supply systems are of central importance for utility vehicles. In particular, the brake system or a pneumatically braked utility vehicle requires compressed air, as do numerous further consumers, for example air suspensions or lift axle devices. The compressed air to be prepared and distributed by the compressed air supply system is delivered by a compressor which is generally driven by the internal combustion engine of the utility vehicle. In a concept which is common in this regard, the compressor is coupled to the internal combustion engine via a pneumatically switchable coupling, the compressed air for the pneumatic activation of the coupling being extracted, in turn, from the compressed air supply system. An example of a system of this type is given in DE 39 23 882 C2.
Owing to the switchability of the coupling, the operation of the compressor can be interrupted, as required, for example during the regeneration phases of the filter unit. The decoupling of the compressor from the internal combustion engine can likewise be used in a directed manner with regard to the energy balance of the utility vehicle.
However, implementing a concept with a switchable coupling is not without problems in all aspects. In particular, high temperatures prevail in the region of the internal combustion engine, so that the switching members, that is to say, in particular, pneumatically and/or electrically activatable valves, have to be designed correspondingly, thus entailing correspondingly high costs.
The object on which the invention is based is to make a available a compressed air supply system with an electrically switchable coupling on the basis of a cost-effective and particularly functionable concept.
The invention builds on the generic compressed air supply system in that, when the valve is being opened or kept open, a pick-up current flows in the magnet coil during a first phase, and, during a second phase, a holding current flows which is lower than the pick-up current a driver signal which causes the holding current being pulse-width-modulated in the second phase. Thus, the power consumption of the magnet coil can be lowered in a controlled way, so that the high temperatures prevailing in the region of the internal combustion engine can be tolerated more effectively. The amplitude of the pulse-width-modulated holding current may be lower than the value of the pick-up current, and, by means of pulse-width modulation, for example with a duty factor of 50%, a further lowering of the heat output emitted by the coil occurs. The pick-up current flows for a period of time of, for example, 50 ms. The duty factor during the pulse-width-modulated holding current phase is, for example, 0.8 to 4 kHz.
The invention is advantageously developed in that, in a third phase, an intermediate current flows which is higher than the holding current. The intermediate current serves for ensuring the open state of the valve. If a lowering of the magnet armature in the direction of its position corresponding to the closed valve position occurs in the holding current phase, then, by means of a suitable intermediate current pulse, the armature can be brought again into a position corresponding to the fully open position. The intermediate current may, for example, have the same value as the pick-up current, and the pulse length may likewise amount, for example, to 50 ms.
Provision can furthermore be made for the valve to be arranged in the region of a fresh air supply to the compressor. In the region of the air supply to the compressor, that is to say, in particular, at the intake connection piece of the latter, comparatively low temperatures prevail, so that a valve can be used, without special account being taken of temperature resistance. In addition, the arrangement mentioned affords the advantage that a particularly short line path may be provided between the valve and the switchable coupling, thus shortening the changeover times of the coupling. This is advantageous particularly in plants which are intended to bring about a frequent changeover of the coupling.
In this respect, there may usefully be provision for the valve to be fastened directly to a housing of the coupling. By the valve being flanged directly on the coupling housing, any line between the valve and the coupling may be dispensed with. A line break is thereby ruled out, and, furthermore, short switching times for the coupling can be ensured.
Furthermore, in terms of the temperature conditions, it is advantageous that the valve is arranged in the region of an air stream which can be generated by the ventilation of the internal combustion engine. The valve is thus additionally cooled. In particular, utilizing the vehicle electronics, the ventilation of the internal combustion engine can be switched on, even if the temperature of the internal combustion engine would not require this. The ventilation then serves mainly for cooling the valve. Further measures which lower the temperature of the magnet coil are good heat transmission from the magnet coil to the valve housing, a cooling duct which runs through the valve and, in particular, may carry intake air of the compressor, and/or cooling ribs on the valve housing.
There is usefully provision for the valve to be an electrically pilot-controllable pneumatic valve. Such a valve, because of the high air throughput achievable, makes it possible to have a rapid pressure build-up at the switching member of the coupling, so that this measure, too, increases the switching speed. Moreover, in comparison with a valve directly controlled electrically, a lower electrical power is required at the same switching capacity.
The compressed air supply system according to the invention may be designed, for example, such that the opening of the valve is caused by an electrical signal from a pressure switch which responds to a pressure in a compressed air preparation plant. One reason for decoupling the internal combustion engine and compressor may be the presence of a sufficient pressure within the compressed air preparation plant or in the region of the consumer connections. It is consequently useful to convert this pressure into an electrical signal which then, in turn, causes the decoupling of the internal combustion engine and compressor. By means of further electrical or electronic components, the driver signal for activating the magnet coil is generated on the basis of the signal generated by the pressure switch.
It is useful, in this connection, that the driver signal is made available by an electronic control which receives pressure-dependent input signals. The signal from the pressure switch is thus first delivered to an electronic control which then, if appropriate, outputs the signal for the valve. The advantage of this is that other parameters present in the utility vehicle can be taken into account in terms of the operation of changing over the coupling.
For example, there may be provision for the electrical driver signal to be made available by an electronic control which receives temperature-dependent input signals. Thus, for example, the temperature in the region of the compressor can be measured, in order, in the case of an increased temperature, to bring about a decoupling of the compressor from the internal combustion engine.
It is likewise possible for the electrical driver signal to be made available by an electronic control which receives input signals dependent on the input-side and/or output-side rotational speed of the coupling. A monitoring of the rotational speeds in the region of the coupling may also be useful, for example with a view to fault diagnosis.
According to a particularly advantageous embodiment of the invention, there is provision for the electronic control to be integrated into a compressed air preparation plant. This can be implemented, in practice, in that the conventional electronic control of the compressed air preparation plant is extended to include the functionality of coupling activation.
It is also conceivable, however, that the electronic control communicates via an interface with a control integrated into a compressed air preparation plant. On this basis, the conventional electronic control of the compressed air preparation plant can remain largely unchanged, and the switching functionality for the coupling can be available externally.
According to a particularly advantageous embodiment of the invention, there is provision for the compressed air delivered to the switch input via the valve to be extracted from a compressed air preparation plant via a nonreturn valve. In order to ensure a high switching speed in the case of a pneumatic valve, delivery to a pneumatic valve must preferably be maintained at a certain pressure level. Consequently, during the opening of the valve, a sufficient pressure is immediately present which can then also quickly cause the changeover of the coupling via the short line to the coupling. Since the compressed air for changing over the coupling is preferably extracted from a compressed air preparation plant which also supplies other consumers with compressed air, a pressure level, sufficient during the ventilating operation, in the line leading to the pneumatic valve is often ensured by an additional pressure reservoir. This is unnecessary if the compressed air delivered to the switch input via the valve is extracted via a nonreturn valve, since the latter prevents a pressure breakdown in the pressure delivery line on account of other effects in the region of the compressed air preparation plant and the consumers connected to it.
According to a preferred embodiment, there may be provision for the nonreturn valve to be integrated into the compressed air preparation plant. An essential constituent of the compressed air preparation plant is a valve housing which has at least one circuit protection device and which makes it possible readily to incorporate a further nonreturn valve by dealing with a merely structural task. However, the external arrangement of the nonreturn valve may also likewise be envisaged.
The invention relates, furthermore, to a method for controlling a compressed air supply system according to the invention and to a utility vehicle having a compressed air supply system according to the invention.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.